Comet 17P/Holmes
Abstract
J. M. Trigo-Rodriguez, Institute of Space Sciences, Consejo Superior de Investigaciones Cientificas and Institut Estudis Espacials de Catalunya, has analyzed infrared images of comet 17P taken with the 1.5-m Carlos Sanchez Infrared Telescope (IAC, Tenerife) by P. Abraham and A. Konkoly (Konkoly Observatory) between Oct. 26.1 and 26.2 UT. An asymmetric distribution of the coma is visible, together with a bright fan of material extending from the nucleus in p.a. 220 deg. Near the false nucleus, the fan is formed by two bright blobs separated by a clear gap. Both features are quickly expanding at an approximate velocity of about 600 m/s. This observation confirms the report by Colas and Lecacheux on CBET 1111 regarding the release of bright expanding blobs. These bright features expand rapidly from the false nucleus, suggesting a massive release of material from a very active region of the cometary surface. The expanding blobs are not seen in filtered B, V, R, and I images taken with the 0.8-m IAC80 Telescope (Tenerife) between Oct. 27.1 and 27.2 by R. Barrena, P. Montanes-Rodriguez, M. F. Nunez, and P. Abraham. A fan extends from the false nucleus in p.a. 220 deg, and the outer coma diameter on those images was 4'.1, or 285000 km. The outer shell of the coma continues expanding on Oct. 30.1 images taken by P. Montanes-Rodriguez; the coma keeps its general asymmetric shape and quite uniform brightness, but its diameter has increased to 7'.1 or 505000 km. A Larson-Sekanina algorithm applied to IAC images reveals several arc-like structures probably produced from active jets, but no clear evidence of fragmentation of the nucleus. J. Lecacheux and F. Colas, Paris Observatory; and B. Gaillard report on images taken on Oct. 28.85 and 31.23 UT at the f/17 focus of the Pic du Midi 1-m telescope under sub-arcsecond conditions: the elongated semi- parallel luminous streaks reported by many observers across the expanding "coma blob" (cf. our previous report in CBET 1111). These streaks are curved dust tails that appear to emanate from about fifteen sites spread around the nuclear condensation, though they are more numerous and brighter in the south- southwest region from the bright condensation; the emanation sites are about 25000 +/- 10 000 km from the condensation (projected on the sky). The set of tails seems to be fan-shaped (divergent) but in a fountain-like pattern. There are three main tails responsible at low resolution for the diffuse light maximum that appears like a "blob". The main nuclear condensation possesses its own dust tail, though it is smaller than the other tails. See http://www.picdumidi.eu/ for the processed images. A. Arai, M. Uemura, M. Sasada, K. S. Kawabata, T. Yamashita, T. Yasuda, R. Matsui, H. Tanaka, O. Nagae, M. Isogai, and T. Ohsugi, Hiroshima University; R. Furusho, Waseda University; J. Watanabe, National Astronomical Observatory of Japan; and M. Kino and S. Sato, Nagoya University, report on behalf of the Kanata observation team that they obtained CCD images of comet 17P/Holmes on Oct. 25.62 (R_c band), 27.66 (I_c), 29.70 (V), and 30.70 UT (I_c) with the KANATA 1.5-m telescope (+ TRISPEC) at Higashi-Hiroshima Observatory. An elongated structure has developed near the nucleus, which consists of a collimated flow and a diffuse expanding cloud embedded in the coma. Preliminary analysis of the last image shows that the position angle of the flow is about 190 deg, being significantly misaligned with that of the cloud (p.a. 210-220 deg). The brightness center of the cloud moved about 33" between Oct. 27 and 30, suggesting a mean velocity of 11"/day (150 m/s at 1.5 AU) on the projected sky. A sample of images can be seen at website URL http://kanatatmp.blogspot.com/search/label/Holmes. Low-resolution spectropolarimetry of the nuclear region on Oct. 25 and 27 showed the comet to be at negative polarization, with preliminary analysis showing the degree of polarization (0.5-0.7 percent) to be smaller than that of a typical comet at similar phase angle. M. Montalto, A. Riffeser, S. Wilke, and U. Hopp, University Observatory, Munich, report that B, V, R, and I imaging with a 0.8-m telescope at Wendelstein Observatory, obtained during Oct. 26-31 show an almost-stellar bright core (with a few arcsec FWHM) and a large cloud that is expanding and fading. The bright central condensation and the centroid of the cloud were separated by 15" on Oct. 27.1 UT, and by 61" on Oct. 31.9; analysis of 103 images shows that the two centers are linearly increasing in distance with a projected velocity of 9".61 +/- 0".4 per day (suggesting that they were coincident on Oct. 25.568 +/- 0.2). Z. Sekanina, Jet Propulsion Laboratory, reports that information on the appearance, morphology, and light curve of the comet allows one to make tentative conclusions on the nature of its megaburst from Oct. 23-24. The brightness data (calibrated by linking K. Kadota's pre- and post-outburst CCD magnitudes with the naked-eye estimates) suggest that, before the event, the comet had been active but fading with heliocentric distance (r) -- from late May following an r**(-n) law with n approximately equal 16. At the event's onset, most probably on Oct. 23.7 +/- 0.2 UT, the comet's total visual magnitude normalized to 1 AU from the earth was 15.3. This is about 30 times brighter than expected for the nucleus based on its diameter of 3.3 km, geometric albedo of 4 percent, and a phase law of 0.035 mag/deg (Lamy et al. 2000, BAAS 32, 1061; Snodgrass et al. 2006, MNRAS 373, 1590). The extremely steeply increasing brightness at the beginning of the megaburst suggests that the rate of dust injection into the coma accelerated with time, or the average particle size was then rapidly decreasing with time (perhaps as a result of runaway particle fragmentation), or both. Indeed, the reported constant rate in mag/hr (IAUC 8886) implies an exponential increase in the cross-sectional area of dust, neglecting the minor contribution from the molecular and atomic species (IAUC 8887).
- Publication:
-
Central Bureau Electronic Telegrams
- Pub Date:
- November 2007
- Bibcode:
- 2007CBET.1118....1T